1. Technical Field
The invention relates to a method for correcting image errors caused by residual magnetism in monitors and other similar display devices based on picture tubes. The invention also relates to an apparatus for correcting image errors caused by residual magnetism in a picture tube.
2. Discussion of Related Art
Picture tubes are apparatus based on cathode ray tubes the purpose of which is to convert picture information in an electrical form into a visible picture. FIG. 1 shows the structure and operating principle of a typical picture tube 10 used in color monitors. The picture tube 10 comprises a neck part 11 and a funnel 12. In addition, the picture tube entity is considered to comprise other components closely related to its operation, such as an electron gun part 13 located inside the picture tube's neck part 11, external components placed on the neck part 11, such as a deflection yoke 20, a convergence coil unit 21 containing the convergence coils, a tilt coil 22, an LMF coil 23, a ring magnet 24, and an R/B convergence coil 27 used in some picture tube models. Additionally, components are found inside the funnel 12, such as magnetic shielding 17, a mask 26, and a screen 25.
Operation of the color picture tube is based on three separate electron beams 14, 15, 16 produced by the electron gun part 13 located inside the neck part. The electron beams 14, 15, 16 travel through a magnetic field produced inside the picture tube by the deflection yoke 20 so that a force is exerted on the electron beams 14, 15, 16 which affects their trajectories. By means of the magnetic field produced by the deflection yoke 20 the electron beams 14, 15, 16 are directed to a desired position on the screen 25. As the electron beams 14, 15, 16 hit the fluorescent surface of the screen 25 they generate a spot of light. By deflecting the electron beams 14, 15, 16 in a rapid pace to various locations on the screen a set of pixels is produced which the eye perceives as a continuous illuminated area.
A mask 26 is placed in front of the screen 25. The mask 26 is made of a thin metal plate having apertures in it. Depending on the type of the picture tube the apertures may be e.g. round holes (so-called shadow mask), narrow slits extending from the bottom of the picture tube to the top (so-called trinitron mask) as in FIG. 2, or elongated in the vertical direction (so-called slot mask).
In a correctly adjusted picture tube the trajectories of all three electron beams 14, 15, 16 intersect at the mask 26 so that they all hit the same aperture 26a in the mask 26, as shown in FIG. 2. Since the beams 14, 15, 16 arrive from slightly different directions, they spread out after the mask 26 to hit their designated phosphor stripes 29a, 29b, 29c. As the beams 14, 15, 16 hit the phosphor stripes 29a, 29b, 29c light is generated the color of which depends on the phosphor used. According to the RGB standard the phosphors are typically chosen such that one produces red (R), the second green (G) and the third blue (B). Since the three spots of light thus generated are close to each other, the eye does not perceive them as separate dots but the visual perception is produced as the sum of all three colors. By varying the relative brightness of the different colors one can produce almost an arbitrary number of different hues.
The information above applies to the operation of a color picture tube in an ideal situation wherein the paths of the electron beams are fully controllable. In practice there may occur inside the picture tube unwanted magnetic fields, too, which cause unwanted deflections in the trajectories of the electron beams 14, 15, 16. This may lead to a point where the electron beams 14, 15, 16 are no longer targeted at the same exact location on the mask 26. If the separations are large compared to the distances between the apertures in the mask 26, the electron beams 14, 15, 16 controlling different color components are directed to different mask apertures instead of the same one, which on the screen causes detachment of the colors from each other. At close inspection the detachment of the colors can be seen in the image, and when viewed from a distance the image appears unsharp.
In monitors and other similar display devices based on picture tubes, image sharpness is one of the most important factors affecting the pleasantness of use of the apparatus. Therefore, attempts have been made to minimize the detachment of colors.
Unwanted magnetic fields are typically caused by residual magnetism in magnetic substances, such as iron, in the picture tube and in its immediate vicinity. Residual magnetism means magnetism that is created in a magnetic substance in a varying magnetic field and which remains after the magnetic field, which caused the magnetization, has been removed.
The picture tube and monitors have several components made of a magnetic material, such as the mask 26 in the picture tube and protective covers made of iron in monitors.
Problems caused by residual magnetism can be eliminated by demagnetizing the magnetized parts, i.e. by removing the magnetism in them.
It is known to direct a strong alternating damped magnetic field to the funnel 12 of the picture tube in order to demagnetize the magnetic shielding 17 and the mask 26. Known prior-art methods apply arrangements in which demagnetization is realized by means of a demagnetizing coil 30 placed in the funnel 12 of the picture tube as shown in FIG. 3. In FIG. 3 the picture tube 10 is viewed from the neck part 11 side, not showing the external structures of the picture tube's neck part.
By means of an alternating damped magnetic field produced by a demagnetizing coil 30 placed in the funnel 12 it has been possible to demagnetize the magnetized components in the funnel 12 of the picture tube.
In addition to a demagnetizing coil 30 placed in the funnel 12 it is known to manufacture a hand-held demagnetizing coil with which it has been possible to demagnetize metal parts in the monitor which, because of their location, remain outside the magnetic field produced by a demagnetizing coil 30 placed in the funnel 12 of the picture tube. As the method based on a hand-held demagnetizing coil requires the purchase of a separate demagnetizing apparatus, the method is used primarily by commercial enterprises assembling and servicing monitors. The application of the method to repeated demagnetization of a monitor in normal use is difficult since in the method the demagnetizing apparatus has to be brought near the components to be demagnetized, which in practice requires that the cover of the monitor be removed.
With the prior-art demagnetizing arrangements based on a demagnetizing coil 30 placed in the funnel 12 of the picture tube it has been possible to eliminate a great part of the image errors caused by residual magnetism. Some of the image errors, however, remain in spite of demagnetization.